Currently, single and two-phase thermal control system used in spacecraft require a mechanical pump to produce the pressure head needed to overcome the loop pressure drop and circulate the working fluid. As power density and operational longevity of spacecraft continue to increase, it is important that spacecraft use a highly reliable and efficient fluid thermal control system. To meet the demand for low-flow thermal control systems, a number of pumps have been adapted from terrestrial application, especially vane and gear pumps. Unfortunately, mechanical pumps have numerous moving parts that can wear out or break. There are moving parts in both the motor and the pump head. In addition, they also require elaborate electronic control circuits that generate heat and are subject to failure. For short duration mission, such as Shuttle flight, this type of mechanical pumps are adequate. However, if a spacecraft is to operate in a microgravity gravity environment for five years without maintenance, then the pump has to be reliable enough to last about 50,000 hours.
Operation of the heat driven pump relies on pressure of vapor in a closed chamber. More specifically heating a liquid contained in a chamber produces a vapor that can be used for pumping function.
Many types of heat driven pumps have been developed in this field. One device includes a chamber which contains a pumping gas to be expanded by heating. A liquid to be pumped is introduced into the chamber through ingress means. Expansion of the gas in response to heating the chamber causes the liquid to exit through egress means. Since there is no means of separating the pumped liquid from the pumping gas, the gas can exit the chamber, reducing performance of the pump.
Another device provides a vapor pressure pump comprising a closed reservoir for liquid, an inlet check valve, an outlet check valve, heating means, and a vapor exhaust valve and a float, both of which are adapted to balance the pressure between the check valves. A liquid introduced into the reservoir moves up the float to close the vapor exhaust valve disposed at top of the reservoir. Vapor generated by heating the reservoir forces the liquid out through the outlet check valve. This device also lacks means for separating the vapor from the liquid to be pumped. In addition, operation of this device relies on a the float, which is a moving part and may become subject to mechanical failure.
Yet another device uses inlet and outlet porous membranes for separating a liquid to be pumped from a pumping vapor. The liquid enters the chamber due to liquid permeability of the inlet porous membrane. Bubbles generated by heating the liquid in the chamber force the liquid to exit through the outlet porous membrane. Since introduction of the liquid relies on capillary effect of the porous membranes, refilling of the pumping chamber would be slow and can result in back flow of the liquid.
A further device provides a heat-driven pump for performing the transport of a liquid by the function of bubbles generated by vaporization and condensation of the liquid under heating. The liquid to be heated for the pumping is in contact with the rest of the liquid in the pumping chamber. Therefore, it would result in heating all the liquid in a pumping chamber to produce bubbles for pumping, lowering efficiency of the pump.
A still further device provides a capillary pumped loop, which comprises a capillary evaporator for vaporizing a liquid refrigerant by absorbing heat, a condenser for turning a vaporized refrigerant into a liquid by transferring heat from the vaporized liquid to a cool object. A wick and a plurality of grooves, both of which are adopted to the present invention, are utilized for pumping.